The environmental and health risks associated with lead have resulted in a comprehensive campaign to eliminate its use in many applications including lead-containing ammunition. In particular, government regulations are forcing a change to lead-free rounds in small arms ammunition because of growing lead contamination problems at firing ranges. Toxic lead-containing dust created by fired rounds poses an air-borne health risk and lead leaching from years worth of accumulated spent rounds is now posing a substantial hazard to local water supplies.
Over the years, a number of composite materials have been proposed as lead substitutes. The methods of making these composites generally involve blending a powdered material having a density greater than that of lead with a powdered binder material having a density less than that of lead. The blended powders are then pressed, injection molded, or extruded to form slugs of the composite material. In order to have acceptable and consistent ballistic properties, the composite material formed after pressing should be void-free (i.e., have a measured density which is about 100% of the theoretical density) and without macroscopic segregation of the components. Also, it is preferred that the composite material should have a density and mechanical properties similar to those of lead so that the composite material may be used as a drop-in replacement for lead-containing ammunition in a wide range of applications.
Most importantly, the composite material should be sufficiently malleable and ductile so that the slugs of the composite material will deform uniformly and allow the composite material to be pressed directly into pointed bullet shapes or to fill the cores of jacketed projectiles.
In order to achieve a density similar to lead, tungsten which has a density of 19.3 g/cm3 has been combined with binder materials such as nylon and tin to make lead-free projectiles. However, the composites made by these methods are either too expensive to manufacture or do not possess one or more of the desired properties, i.e., ductility, malleability, density, etc.
More particularly, tungsten-nylon composites are 50% more expensive than lead because of the high tungsten content needed to achieve a lead-like density. And, even at the highest tungsten content possible for these composites, about 96 wt. % W, the density of a tungsten-nylon composite is 10.8 g/cm3 or only about 95% that of lead.
Although less expensive than tungsten-nylon, tungsten-tin composites have experienced greater problems with achieving lead-like properties. For example, U.S. Pat. No. 5,760,311 to Lowden et al. describes a tungsten-tin (W—Sn) composite made by blending large tungsten particulates (149 μm or greater) with a tin powder in either a 58/42 or 70/30 weight ratio of tungsten to tin. The blended powder was compressed at pressures ranging from 140 to 350 MPa to form slugs having densities ranging from 9.76 to 11.49 g/cm3. The compressive strengths of the slugs ranged from 70 to 137 MPa which is significantly higher than that of lead (about 20 MPa). This means that the slugs would not have sufficient malleability to be pressed directly into bullet shapes or uniformly deform to fill the core of a jacketed projectile. Moreover, the slugs could only be pressed to between about 89% (70/30 blend) to 92% (58/42 blend) of theoretical density meaning that the slugs contained a significant quantity of void space. The existence of a significant quantity of voids in the material may result in an inhomogeneous density in the projectile which can affect its ballistic performance and, in particular, its accuracy. Furthermore, the highest densities could be achieved only by pressing the blends at pressures of 280 Mpa or greater.